Use 'project-file as category metadata in completing read

project-file is a more suitable category for the completion (also used
by project.el).

'vertico-prescient-completion-category-overrides changes the
completion style for 'file to '(basic partial-completion) and
limits completion to prefix matches.

Fixes #1902

projectile.el

@@ -2079,7 +2079,7 @@ project-root for every file."
                                                  ;; embark to enhance how they
                                                  ;; present candidates
                                                  ((eq action 'metadata)
-                                                  '(metadata . ((category . file))))
+                                                  '(metadata . ((category . project-file))))
                                                  (t
                                                   (complete-with-action action choices string pred))))
                                        nil nil initial-input))